1. Field of the Invention
The present invention relates generally to the field of optical fiber cables and more particularly to a spliced optical fiber cable having at least one drop cable extending therefrom.
2. Description of the Prior Art
With the advent of local area networks and the relative broadband capabilities of fiber optic links, it has become commonplace for new communication systems to include fiber optic capabilities. In the use of optical fibers, optical fiber cables are provided for physical protection of the fibers in view of the fragile nature of the glass optical fibers. A main distribution or trunk cable, having a predetermined number of optical fibers, is normally used between two termination points in a communication system. At a point where a connection must be made to local homes or buildings, a splice is made to one or more optical fibers in the distribution cable for connecting a separate smaller sized stub cable, or drop cable. The drop cable is a small flexible cable and is separately routed to the local home or building.
In those systems in which drop cables are required to be spliced to the distribution cable, the required splice must be made on site. This is a very difficult and time-consuming procedure which entails accessing the cable, carefully opening the outer sheath, radial strength yarns, buffer tubes, etc., for accessing the specified optical fibers, splicing the specified optical fibers to the drop cable optical fibers, and sealing the spliced point. Additionally, the technician making the splice may be faced with such adverse working conditions as extreme weather or inconvenient cable placement.
The splice, or branch point of the optical fiber cable must have a number of features. First, it must protect the optical fibers and splices from moisture and mechanical damage and provide an adequately strong anchoring point for the drop cable. It also must insure that the electrical continuity of every metallic cable component is maintained and insure that the minimum optical fiber bend radius is not violated. The branch point should also be reenterable for future optical fiber branching. Thus, to provide a cable with a branch point having these features, a splice closure is used.
Conventional splice closures are normally comprised of metal or plastic and are utilized to enclose the splice thereby protecting the optical fibers and splices from environmental damage. The splice closures provide a strong anchoring point for the drop cables so that if tensile forces are applied to the drop cables, the splices will not be affected. In addition, the splice closure comprises a large cavity therein for providing space for making the splices and for providing slack buffer tube and optical fiber storage space.
Conventional splice closures are either the "butt-end-type" or the "through-put-type" splice closure. Alternatively, the splice closure may be a composite of the two types being adaptable for either splice method.
The butt-end-type splice closure is configured so that the distribution cable enters and exits the splice closure from the same end. In contrast, the through-put-type splice closures are configured so that the distribution cable enters the closure at one end and exits at the other.
In both types of splice closures, however, the manner in which the splice is made is substantially the same. Within the splice closure, the buffer tubes of the distribution cable are accessed and are coiled a number of times so that an adequate amount of optical fiber is available for splicing. A portion of the buffer tube(s) encompassing the specified optical fibers is removed (approximately 24 inches) so that the specified optical fibers may be cut and prepared for splicing with the corresponding drop cable optical fibers. Although not all of the 24 inches of optical fiber is required for the splice, the slack optical fiber is desired so that splices may be redone if necessary or if future optical fiber branching is desired.
In a similar manner, the drop cable (or cables), is prepared for splicing with the specified distribution cable optical fibers. After the splice is made using well-known splicing methods, the splice is held in a splice tray which is mounted within the splice closure cavity. The splice tray prevents a splice from moving about within the closure upon closure movement. The splice closure is assembled by placing the coiled buffer tubes and slack optical fibers within the cavity and enclosing the cavity so that the splice is completely encapsulated.
Because the conventional splice closure must house the slack buffer tubes and optical fibers without violating the minimum bend radius of the optical fibers, the splice closure cavity must be somewhat large to provide adequate space for storage and splicing purposes. Ultimately, this results in a splice closure which is large and bulky, thus making it impractical for all other uses except for on-site optical fiber splicing.
There have been two solutions developed by the assignee of the present invention which pertain to the branching of such cables. In U.S. Pat. No. 5,121,458 issued Jun. 9, 1992 entitled "Preterminated Fiber Optic Cable", a water-resistent closure less than 4 inches in diameter stores and protects the optical fiber and splices which make-up a branch from the distribution or trunk cable to the drop or stub cable(s). The branch can be the origination point for up to six drop cables, and the cable composition can be either all fiber, or composite fiber/metallic conductor. The splice closure can accommodate a relatively large number of fiber optic splices (approximately 12), and can efficiently store around 24 inches of excess fiber. The maximum length of the drop (or stub) cable is essentially unlimited, but typically is less than 100 feet.
Although the assembly described in U.S. Pat. No. 5,121,458 has a great deal of flexibility in regards to number of drops (or stubs), number of spliced fibers, and length of drop cables, the outer diameter of slightly less than 4 inches limits installation in or through typical duct systems which are usually less than 2.5 inches in inner diameter.
In U.S. Pat. No. 5,125,060 issued Jun. 23, 1992 entitled "Fiber Optic Cable Having Spliceless Fiber Branch and Method of Making", a water-resistent branch less than 11/4 inches in diameter acts as a transition from the distributor trunk cable to the drop or stub cable. Instead of the branch containing fiber optic splices, the branched fibers are cut no more than 12 feet upstream, pulled out of the distribution cable, and placed into the protective drop cable structure. Water ingression is prevented by covering the branch with an adhesive lined heat-shrinkable plastic.
The spliceless fiber branch of U.S. Pat. No. 5,125,060 overcomes the diameter limitation found in U.S. Pat. No. 5,121,458, but the maximum length of the drop cables is practically limited to 12 feet. In addition, two trunk cable sheath entries are required to access and branch the optical fiber(s) which increases installation time, cost and the reliability and water ingression risk.
Thus, it would be desirable to develop a solution to the shortcomings found in the prior art.